Data reproduction control method and apparatus, and optical disk unit

ABSTRACT

The present invention realizes stable and more proper data reproduction even when a reproduced signal varies locally due to a noise or includes a steady distortion. In a data reproduction control method and apparatus, and an optical disk unit that adjust sampled values, in sampling a signal reproduced from a recording medium recorded with data, the frequencies of the levels of quantized data are counted, a histogram of the frequencies of the levels is created based on the counting results, and more accurate reproduced data is obtained by means of a distribution of the frequencies of the levels in the histogram.

This is a divisional of application Ser. No. 09/900,790, filed Jul. 6,2001 now U.S. Pat. No. 6,618,338, which is a continuation ofInternational Application No. PCT/JP99/04490, filed Aug. 20, 1999.

TECHNICAL FIELD

The present invention relates to a data reproduction control method andapparatus that control a data reproduction characteristic based on thedistribution characteristic of quantized data obtained by quantizing asignal reproduced from a recording medium.

The present invention further relates to an optical disk unit thatoperates in accordance with such a data reproduction control method.

BACKGROUND ART

Recently, optical disk units, magneto-optical disk units, and magneticdisk units that are employed as auxiliary storages of a computer haverequired a data recording and reproduction method with higher accuracyin order to meet a demand for increase in the recording density of arecording medium.

Conventionally, as a method of recording data on and reproducing datafrom optical disk units, magneto-optical disk units, and magnetic diskunits with high accuracy, the PRML (partial response maximum likelihood)technique has been proposed. By this PRML technique, a recording datasignal is modulated into a so-called partial response (PR) waveform tobe recorded on a recording medium, while a reproduced signal from therecording medium is sampled at regular intervals to be quantized so thatthe quantized data is subjected to waveform equalization to correspondto the partial response (PR) waveform. The reproduced signal may besampled at regular intervals and quantized after beingwaveform-equalized to correspond to the partial response (PR) waveform.Then, from the resulting quantized data, data of maximum likelihood isdetermined in accordance with a maximum likelihood (ML) detectiontechnique such as the so-called Viterbi detection.

The conventional PRML technique employs a(n) (adaptive equalization)technique that controls the characteristic of waveform equalization soas to minimize a difference between quantized data obtained by samplingand a value that is expected to be obtained from a partial response (PR)waveform. By thus controlling the characteristic of waveformequalization, quantized data corresponding to a signal waveform moresimilar to the partial response (PR) waveform can be obtained.Consequently, data reproduction can be performed with more accuracy.

However, such a technique to control the characteristic of waveformequalization as described above controls the equalization characteristicbased on a local state of the reproduced signal. Therefore, if thereproduced signal varies locally due to a noise, stable and more properdata reproduction cannot be achieved.

Further, in order to realize high-density data recording, a mediumhaving a magnetically induced super resolution effect (an MSR (MagneticSuper Resolution) medium), for instance, is about to be put intopractical use. In this MSR medium, a super resolution effect is producedby forming a mask using the heat distribution of a light beam, so that anonlinear distortion component is generated in the reproduced signal bya non-uniform heat distribution of the light beam moving on the medium.This nonlinear distortion component distorts the reproduced waveform.

This distortion appears as a droop phenomenon that occurs notably at atrailing or rising edge of the reproduced signal. Further, thisdistortion is not generated only at a given point, but is generatedsteadily.

Thus, there has been proposed a technique to reproduce two clock signalssynchronizing with the trailing and rising edges of the reproducedsignal, respectively, (PLL) and sample the reproduced signal by usingthe two clock signals (DUAL PLL system).

Even if the reproduced signal is distorted at a trailing or rising edgethereof, sampling can be performed at a timing corresponding to thedistortion by sampling the reproduced signal by such clock signalssynchronizing with the trailing and rising edges of the reproducedsignal, respectively. Therefore, correct output data can be obtained.

However, the above-described DUAL PLL system has a complicated circuitstructure because of its employment of the two clock signals to samplethe reproduced signal. On the other hand, there is another method ofsampling the reproduced signal which method employs a clock signal thatfacilitates circuit simplification (SINGLE PLL system). However, theSINGLE PLL method is prevented from obtaining correct output data in thecase of sampling such a drooping waveform as described above.

DISCLOSURE OF THE INVENTION

It is a general object of the present invention to provide a useful datareproduction control method and apparatus, and a useful optical diskunit that are improved to eliminate the above-described disadvantage.

An object of the present invention is to provide a data reproductioncontrol method and apparatus that can reproduce data stably and moreproperly with as simple a circuit structure as possible even when areproduced signal varies locally due to a noise to be distorted orincludes a steady distortion at a trailing or rising edge thereof.

A more specific object of the present invention is to provide a datareproduction control method and apparatus that can reproduce data stablyand more properly even when a reproduced signal varies locally due to anoise.

The above objects of the present invention are achieved by a datareproduction control method controlling a distribution characteristic ofquantized data by adjusting a waveform equalization characteristic at atime of performing quantization and waveform equalization on a signalreproduced from a recording medium recorded with data in accordance witha given recording code, wherein frequencies of levels of the quantizeddata are counted, and the waveform equalization characteristic isadjusted so that a distribution of the counted frequencies of the levelsapproaches a distribution that should be obtained based on the givenrecording code.

The above objects of the present invention are also achieved by a datareproduction control apparatus controlling a distribution characteristicof quantized data by adjusting a waveform equalization characteristic ofa waveform equalizer when a signal reproduced from a recording mediumrecorded with data in accordance with a given recording code issubjected to operations in quantization means and the waveformequalizer, which data reproduction control apparatus includes countingmeans for counting frequencies of levels of the quantized data, andequalization characteristic adjustment means for adjusting the waveformequalization characteristic of the waveform equalizer so that adistribution of the frequencies of the levels obtained in the countingmeans approaches a distribution that should be obtained based on thegiven recording code.

According to the above-described method and apparatus of the presentinvention, by setting a period of signal reproduction relatively long,the frequencies of the levels of quantized data are counted based on alarger amount of quantized data. Waveform equalization characteristic isadjusted based on a distribution of the thus obtained frequencies of thelevels of the quantized data. As a result, the waveform equalizationcharacteristic is adjustable so that a more proper waveform equalizationoutput can be obtained even if a reproduced signal varies locally.

In another mode of the present invention, the frequencies of the levelsof the quantized data may be counted at given intervals in the period ofsignal reproduction from the recording medium.

In yet another mode of the present invention, a given number of periodsof signal reproduction from the recording medium may be set, and thefrequencies of the levels of the quantized data may be counted and thewaveform equalization characteristic may be adjusted in each of theperiods of signal reproduction.

In another mode of the present invention, the waveform equalizationcharacteristic may be adjusted by a given amount in each of the periodsof signal reproduction.

In yet another mode of the present invention, the waveform equalizationcharacteristic may be adjusted by a first given amount in first givenones of the periods of signal reproduction, and be adjusted by a secondgiven amount less than the first given amount in the rest of the periodsof signal reproduction.

In yet another mode of the present invention, a discrete state in thedistribution of the counted frequencies of the levels may be determined,and the waveform equalization characteristic may be adjusted so that thediscrete state may match a discrete state of the distribution thatshould be obtained based on the given recording code.

Yet another object of the present invention is to provide a datareproduction control method and apparatus that realizes stable and moreproper data reproduction even if a reproduced signal includes a steadydistortion.

The above objects of the present invention are achieved by a datareproduction control method performing quantization on a signalreproduced from a recording medium recorded with data in accordance witha given recording code, and controlling a data reproductioncharacteristic at a time of determining, by a maximum likelihooddetection process corresponding to the recording code, reproduced datafrom quantized data obtained by the quantization, wherein frequencies oflevels of the quantized data are counted, and a processingcharacteristic in the maximum likelihood detection process is adjustedbased on a distribution of the counted frequencies of the levels.

In another mode of the present invention, in the above-described datareproduction control method, the maximum likelihood detection processmay be a Viterbi decoding algorithm, and expected values employed in theViterbi decoding algorithm may be provided based on the distribution ofthe counted frequencies of the levels.

According to this data reproduction control method, even if a reproducedsignal includes a steady distortion, an operation following a maximumlikelihood detection process is performed with a processingcharacteristic matching the distortion. Therefore, even if thereproduced signal includes a distortion, stable and more proper datareproduction can be realized.

In yet another mode of the present invention, in the above-describeddata reproduction control method, a trailing edge of the reproducedsignal may be detected based on a level change of the quantized data,the frequencies of the levels of the quantized data may be counted atthe detected trailing edge of the reproduced signal, and the processingcharacteristic in the maximum likelihood detection process for thetrailing edge of the reproduced signal may be adjusted based on thedistribution of the counted frequencies of the levels.

In yet another mode of the present invention, in the above-describeddata reproduction control method, a rising edge of the reproduced signalmay be detected based on a level change of the quantized data, thefrequencies of the levels of the quantized data may be counted at thedetected rising edge of the reproduced signal, and the processingcharacteristic in the maximum likelihood detection process for therising edge of the reproduced signal may be adjusted based on thedistribution of the counted frequencies of the levels.

According to these data reproduction control methods, when a reproducedsignal includes a distortion especially at its trailing or rising edgedue to a characteristic of a recording medium, a processingcharacteristic in a maximum likelihood detection process for the edge isindividually adjustable.

The above objects of the present invention are also achieved by a datareproduction control apparatus performing quantization on a signalreproduced from a recording medium recorded with data in accordance witha given recording code, and controlling a data reproductioncharacteristic when maximum likelihood detection means, in accordancewith a maximum likelihood detection process corresponding to therecording code, determines reproduced data from quantized data obtainedby the quantization, which data reproduction control apparatus includescounting means for counting frequencies of levels of the quantized data,and characteristic adjustment means for adjusting a processingcharacteristic in the maximum likelihood detection means based on adistribution of the frequencies of the levels obtained in the countingmeans.

In another mode of the present invention, in the above-described datareproduction control apparatus, the maximum likelihood detection meansmay be Viterbi detection means by which the reproduced data isdetermined in accordance with a Viterbi decoding algorithm, and thecharacteristic adjustment means may include expected value setting meansfor setting expected values employed in the Viterbi detection meansbased on the distribution of the counted frequencies of the levels.

In yet another mode of the present invention, the above-described datareproduction control apparatus may include trailing edge detection meansfor detecting a trailing edge of the reproduced signal based on a levelchange of the quantized data, and counting control means for enablingthe counting means while the trailing edge detection means detects thetrailing edge of the reproduced signal, wherein the characteristicadjustment means may adjust the processing characteristic in the maximumlikelihood detection means for the trailing edge of the reproducedsignal based on the distribution of the frequencies of the levelsobtained in the enabled counting means.

In yet another mode of the present, invention, the above-described datareproduction control apparatus may include rising edge detection meansfor detecting a rising edge of the reproduced signal based on a levelchange of the quantized data, and counting control means for enablingthe counting means while the rising edge detection means detects therising edge of the reproduced signal, wherein the characteristicadjustment means may adjust the processing characteristic in the maximumlikelihood detection means for the rising edge of the reproduced signalbased on the distribution of the frequencies of the levels obtained inthe enabled counting means.

Yet another object of the present invention is to provide an opticaldisk unit that operates in accordance with the above-described datareproduction control methods.

The above object of the present invention is achieved by an optical diskunit having a reproduction system including processing means forperforming quantization and waveform equalization on a signal reproducedfrom an optical disk medium recorded with data in accordance with agiven recording code, and maximum likelihood detection means fordetermining, in accordance with a maximum likelihood detection processcorresponding to the recording code, reproduced data fromwaveform-equalized quantized data obtained in the processing means,which optical disk unit includes counting means for counting frequenciesof levels of the quantized data, and equalization characteristicadjustment means for adjusting a waveform equalization characteristic inthe waveform equalization so that a distribution of the frequencies ofthe levels obtained in the counting means approaches a distribution thatshould be obtained based on the given recording code.

The above object of the present invention is also achieved by an opticaldisk unit having a reproduction system including quantization means forperforming quantization on a signal reproduced from an optical diskmedium recorded with data in accordance with a given recording code, andmaximum likelihood detection means for determining, in accordance with amaximum likelihood detection process corresponding to the recordingcode, reproduced data from quantized data obtained in said quantizationmeans, which optical disk unit includes counting means for countingfrequencies of levels of the quantized data, and characteristicadjustment means for adjusting a processing characteristic in themaximum likelihood detection means based on a distribution of thefrequencies of the levels obtained in the counting means.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings.

FIG. 1 is a block diagram showing a structure of a magneto-optical diskto which a reproduced signal control method and apparatus according toan embodiment of the present invention are applied;

FIG. 2 is a diagram showing a structure of a read system;

FIG. 3 is a diagram showing a structure of a digital equalizer;

FIG. 4 is a block diagram showing a detailed structure of an equalizercontrol unit shown in FIG. 2;

FIG. 5 is a diagram showing a structure of a decoder of the equalizercontrol unit;

FIG. 6 is a diagram showing a structure of each counter in a countingpart of the equalizer control unit;

FIG. 7 is a block diagram showing a detailed structure of a histogramoperation enabling part of the equalizer control unit;

FIG. 8 is a diagram showing a detailed structure of a timing controlpart of the equalizer control unit;

FIG. 9 is a timing chart showing a signal state of each part in thehistogram operation enabling part and the timing control part of theequalizer control unit;

FIG. 10 is a diagram showing a reproduced signal;

FIG. 11 is a diagram showing a histogram of sampled values of thereproduced signal shown in FIG. 10;

FIG. 12 is a flowchart showing steps of an adjustment operation of tapcoefficients of the digital equalizer;

FIG. 13 is a flowchart showing detailed steps of a recalculationoperation of the tap coefficients in the operation of FIG. 12;

FIG. 14 is a diagram showing another example of the reproduced signal;

FIG. 15 is a diagram showing a histogram of sampled values of thereproduced signal shown in FIG. 14;

FIG. 16 is a diagram showing a structure of the read system;

FIG. 17 is a block diagram showing a detailed structure of an expectedvalue setting unit;

FIG. 18 is a diagram showing structures of the digital equalizer and atrailing edge detection part;

FIG. 19 is a diagram showing a trailing edge condition of a waveform;

FIG. 20 is a diagram showing a peak time condition of a waveform;

FIG. 21 is a diagram showing timing of trailing edge detection and anequalizer output;

FIG. 22 is a diagram showing timing of an equalizer output waveform andan output of the trailing edge detection part;

FIG. 23 is a diagram showing a structure of a rising edge detectionpart;

FIG. 24 is a diagram showing a rising edge condition of a waveform;

FIG. 25 is a diagram showing a bottom time condition of a waveform;

FIG. 26 is a diagram showing an ideal reproduced signal;

FIG. 27 is a diagram showing a histogram of sampled values of thereproduced signal shown in FIG. 26;

FIG. 28 is a diagram showing an example of the reproduced signal;

FIG. 29 is a diagram showing a histogram of sampled values of thereproduced signal shown in FIG. 28;

FIG. 30 is a diagram showing a reproduced signal with droops;

FIG. 31 is a diagram showing a histogram of sampled values of thereproduced signal shown in FIG. 30;

FIG. 32 is a diagram showing a reproduced signal with droops andoffsets;

FIG. 33 is a diagram showing a histogram of sampled values of thereproduced signal shown in FIG. 32;

FIG. 34 is a diagram showing the reproduced signal with droops;

FIG. 35 is a diagram showing a histogram of sampled values of thereproduced signal shown in FIG. 34 in a case where the trailing edgedetection part is employed;

FIG. 36 is a diagram showing the reproduced signal with droops;

FIG. 37 is a diagram showing a histogram of sampled values of thereproduced signal shown in FIG. 36 in a case where the rising edgedetection part is employed;

FIG. 38 is a diagram showing waveforms of quantized data and expectedvalues;

FIG. 39 is a diagram showing an expected value setting; and

FIG. 40 is a diagram showing a structure of a Viterbi detector.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, a description will be given, with reference to the drawings, ofembodiments of the present invention.

An optical disk unit according to an embodiment of the present inventionhas a structure shown in FIG. 1. This optical disk unit is amagneto-optical disk unit that employs a magneto-optical disk as arecording medium.

In FIG. 1, this magneto-optical disk unit includes a magneto-opticaldisk 10 that serves as a recording medium, an optical head 20, anamplifier 21, a read system unit 25, a write system unit 26, anelectromagnet 27, a control unit 28, a servo system unit 29, and a motor30. Data is recorded on the magneto-optical disk 10 in accordance with apredetermined recording code (for instance, partial response PR(1, 1) ofa constraint length 2), and data is read out from the magneto-opticaldisk 10.

The control unit 28 controls the read system unit 25, the write systemunit 26, and the servo system unit 29 in accordance with read-data andwrite-data commands supplied via a connector 32 and an interface circuit31 from an external unit. When the read-data command is supplied, theoptical head optically scans the magneto-optical disk 10 to output areproduced signal, which is supplied via the amplifier 21 to the readsystem unit 25. The read system unit 25 quantizes the suppliedreproduced signal and generates output data in accordance with a maximumlikelihood (ML) decoding algorithm (for instance, the Viterbi decodingalgorithm). The output data is supplied to the control unit 28 and isoutput therefrom to the external unit via the interface circuit 31 andthe connector 32.

When the control unit 28 receives the write command together withrecording data, the control unit 28 modulates the recording data inaccordance with the predetermined recording code (for instance, partialresponse PR(1, 1)), and supplies the modulated data to the write systemunit 26. The write system unit 26 controls the optical head 20 inaccordance with the supplied data, and further controls theelectromagnet 27 based on data obtained from the modulation of therecording data. As a result, the data is written to the magneto-opticaldisk 10 in accordance with the above-described predetermined recordingcode.

The servo system unit 29 controlled by the control unit 28 drives themotor 30 to rotate the magneto-optical disk 10 at a predetermined speed,and places the optical head 20 in a recording or reproduction positionon the magneto-optical disk 10.

The above-described read system unit 25 has a structure shown in FIG. 2.

In FIG. 2, the read system unit 25, includes a low-pass filter (LPF) 22,an analog-to-digital converter (ADC) 23, a digital equalizer (EQ) 24, aViterbi detector 100, and an equalizer control unit 200. The reproducedsignal amplified by the amplifier 21 (see FIG. 1) is waveform-shaped bythe low-pass filter (LPF) 22. The analog-to-digital converter (ADC) 23synchronizes the waveform-shaped reproduced signal with a given clock(not shown) and converts the reproduced signal into a digital signal.That is, the reproduced signal is quantized by being sampled insynchronism with a clock signal.

A sampled value supplied from the analog-to-digital converter (ADC) 23is waveform-equalized by the digital equalizer (EQ) 24 to be output asequalized output data (quantized data). The equalized output datasupplied from the digital equalizer (EQ) 24 is supplied to the Viterbidetector 100 in synchronism with the above-described clock signal. TheViterbi detector 100 detects recorded data in accordance with theViterbi decoding algorithm from the equalized output data sequentiallysupplied in synchronism with the clock signal, and outputs the recordeddata as reproduced data.

The output of the digital equalizer 24 is connected to the equalizercontrol unit 200. The equalizer control unit 200 controls theequalization characteristic of the digital equalizer 24 based on thestate of the equalized output data sequentially output from the digitalequalizer 24 at a time of, for instance, reproducing a signal from themagneto-optical disk 10 in an adjustment mode.

The digital equalizer 24 is formed of a transversal filter shown in FIG.3. That is, the digital equalizer 24 includes a series of five cascadeddelay elements (registers) T, and adds up (+) values obtained bymultiplying the outputs of the delay elements by tap coefficients k−1,k−2, k0, k+1, and k+2, respectively. The equalization characteristic ofthe digital equalizer 24 can be controlled by adjusting the tapcoefficients k−1, k−2, k0, k+1, and k+2.

The equalizer control unit 200 has a structure shown in FIG. 4.

In FIG. 4, the equalizer control unit 200 includes a decoder 202, acounting part 204, a histogram memory 206, an MPU 210, a ROM 212, anonvolatile memory 214 (for instance, a flash ROM), a setting memory216, a histogram operation enabling part 218, and a timing control part220. The MPU 210, ROM 212, nonvolatile memory 214, histogram memory 206,and setting memory 216 are connected by a bus 208 (including a data bus,an address bus, and a control line).

The MPU 210 controls the whole equalizer control unit 200 and performs alater-described determination operation of the tap coefficients of thedigital equalizer 24 in accordance with programs stored in the ROM 212.The nonvolatile memory 214 retains the values of the tap coefficientsk−1, k−2, k0, k+1, and k+2 determined by the tap coefficientdetermination operation of the MPU 210. The setting memory 216, based ona command supplied from the MPU 210, stores set values used in the tapcoefficient determination operation and the tap coefficients to besupplied to the digital equalizer 24. The setting memory 216 also storesreset information (reset) to the counting part 204.

The decoder 202 classifies the equalized output data from the digitalequalizer 24 by level. In this case, the analog-to-digital converter(ADC) 23 outputs the 6-bit quantized data, and the digital equalizer 24also outputs the 6-bit equalized output data. The decoder 202 to whichsuch equalized output data is input includes a decoder that decodes a6-bit input into a 64-bit output. That is, each output bit of thedecoder 202 corresponds to any of 64 (0 through 63) levels representedby the 6-bit equalized output data (64 classifications).

The counting part 204 includes 64 counters 204(1) through 204(64) eachconnected with a corresponding one of the output bits of the decoder202. Each counter 204(i) (i=1, 2, . . . , 64) has a structure shown inFIG. 6. That is, each counter 204(i) includes a clock terminal (clk), anenabling terminal (en), and a CLEAR terminal (clr), and counts clockpulses input to the clock terminal (clk) when the enabling terminal (en)is in an enabled state (for instance, HIGH).

The synchronizing clock signal supplied to the analog-to-digitalconverter (ADC) 23 quantizing the above-described reproduced signal isinput to the clock terminal (clk), and a corresponding output bit signalfrom the decoder 202 is input to the enabling terminal (en) via an ANDelement 205 that is set to an enabled state by an enabling signal (histoenable) output from the histogram operation enabling part 218 asdescribed later. A reset signal based on the above-described resetinformation stored in the setting memory 216 is input to the CLEARterminal (clr).

Since the equalized output data is input to the decoder 202 from thedigital equalizer 24 in synchronism with the above-describedsynchronizing signal, each counter 204(i) having the above-describedstructure counts the frequency of the corresponding level of theequalized output data.

The count value of each counter 204(i) of the counting part 204 issupplied to the histogram memory 206. As a result, the histogram memory206 stores the frequency of each of the levels (0 through 63) of theequalized output data.

The output bit number X of each counter 204(i) determines the maximumvalue of the frequency of each corresponding classified level.

The histogram operation enabling part 218 generates the enabling signal(histo enable) for enabling a count operation (histogram creationoperation) in the counting part 204 based on the set values stored inthe setting memory 216 and a timing signal (restart pulse) supplied fromthe timing control part 220. The histogram operation enabling part 218has a structure shown in FIG. 7. In FIG. 7, the histogram operationenabling part 218 includes a first counter 218 a for detecting an upperlimit value, an OR element 218 b, a flip-flop (JK-FF) 218 c, and asecond counter 218 d for detecting an upper limit value. Although aclock terminal is omitted for simplification purposes, clock pulses aresupplied to the counters and the flip-flop.

The first counter 218 a includes a load value terminal (load) forsetting the upper limit value and an enabling terminal (en). A startcount value corresponding to a start time of the above-describedhistogram creation operation is input to the load value terminal (load).The start count value is processed in the MPU 210 to be stored in thesetting memory 216, and is supplied therefrom to the first counter 218a. A read-enabling signal (read gate) that is made valid at a time ofreading data from the magneto-optical disk 10 is input to the enablingterminal (en). The first counter 218 a starts performing a countoperation on a predetermined clock signal, which may be the same as theclock signal used for sampling the reproduced signal, from a data readstart time, and outputs a start pulse when its count value reaches thestart count value.

The flip-flop (JK-FF) 218 c includes J and K terminals, and a CLEARterminal (CL). The start pulse supplied from the first counter 218 c isinput to the J terminal via the OR element 218 b, and the read-enablingsignal (read gate) is input to the CLEAR terminal (CL). A stop pulse,which is the output of the second counter 218 d, is input to the Kterminal. By the above-described structure, when the read-enablingsignal (read gate) is made valid (when the data readout starts), theenabling signal (histo enable) that is valid from the output of thestart pulse from the first counter 218 a to the output of the stop pulsefrom the second counter 218 d is output from the flip-flop (JK-FF) 218c.

The second counter 218 d also includes a load value terminal (load) forsetting the upper limit value and an enabling terminal (en). A stopcount value corresponding to an end time of the above-describedhistogram creation operation is input to the load value terminal (load).The stop count value is also processed in the MPU 210 to be stored inthe setting memory 216, and is supplied therefrom to the second counter218 d. The enabling signal (histo enable) supplied from the flip-flop(JK-FF) 218 c is input to the enabling terminal (en). The second counter218 d starts performing a count operation on a predetermined clocksignal, which may be the same as the clock signal used for sampling thereproduced signal, after the enabling signal (en) for enabling the countoperation (histogram creation operation) in the counting part 204 ismade valid, and outputs a stop pulse when its count value reaches theend count value.

Thus, the histogram operation enabling part 218 having theabove-described structure outputs the enabling signal (histo enable)that is valid from the start time corresponding to the start count valueto the end time corresponding to the end count value.

The timing control part 220 supplying the restart pulse to the histogramoperation enabling part 218 has a structure shown in FIG. 8. In FIG. 8,the timing control part 220 includes an operation interval control part220 a, an AND element 220 b, a rising edge detection part 220 c, acounter 220 d for detecting an upper limit, a first flip-flop (JK-FF)220 e, and a second flip-flop (JK-FF) 220 f. A clock terminal is alsoomitted in FIG. 8 for simplification purposes as in FIG. 7.

The first flip-flop (JK-FF) 220 e includes J and K terminals, and aCLEAR terminal (CL). The enabling signal (histo enable) supplied fromthe histogram operation enabling part 218 is input to the J terminal,which is always maintained at a LOW level (“0”). The read-enablingsignal (read gate) is input to the CLEAR terminal (CL). By thisstructure, the first flip-flop (JK-FF) 220 e outputs a first controlsignal that is valid from when the enabling signal (histo enable) ismade valid for the first time to when the read-enabling signal (readgate) is made invalid.

The operation interval control part 220 a has a counter function, andoutputs a pulse signal when its count value reaches a set upper limitvalue. The operation interval control part 220 a includes a load valueterminal (load) for setting the upper limit value and an enablingterminal (en). The inverted signal of the enabling signal (histo enable)supplied from the histogram operation enabling part 218 is input to theenabling terminal (en) via the AND element 220 b controlled by the firstcontrol signal supplied from the first flip-flop (JK-FF) 220 e and asecond control signal supplied, as described later, from the secondflip-flop (JK-FF) 220 f. An invalidity count value corresponding to atime to stop the histogram creation operation (an invalidity time) isinput to the load value terminal. This invalidity count value isprocessed in the MPU 210 to be stored in the setting memory 216, and issupplied therefrom to the operation interval control part 220 a. Theoperation interval control part 220 a starts performing a countoperation on a predetermined clock signal, which may be the same as theclock signal used for sampling the reproduced signal, after the enablingsignal (histo enable) supplied from the histogram operation enablingpart 218 is made invalid (, or the inverted signal is made valid), andoutputs the above-mentioned pulse, that is, a restart pulse, when itscount value reaches the invalidity count value. The restart pulse issupplied to the histogram operation enabling part 218.

Back in FIG. 7, in the histogram operation enabling part 218, thisrestart pulse is supplied via the OR element 218 b to the J terminal ofthe flip-flop (JK-FF) 218 c to make the enabling signal (histo enable)valid just as the start signal.

Therefore, by the above-described structure, after a given period oftime (corresponding to the invalidity count value) passes since theenabling signal (histo enable) made valid by the start pulse is madeinvalid by the stop signal, the enabling signal (histo enable) is madevalid again by the restart pulse.

In FIG. 8, the rising edge detection part 220 c detects a rising edge ofthe enabling signal (histo enable) supplied from the histogram operationenabling part 218 to output a rising edge detection signal. The counter220 d includes a load value terminal (load) and an enabling terminal(en). The number of times the histogram creation operation is performedis input to the load value terminal (load). This number of times isprocessed in the MPU 210 to be stored in the setting memory 216, and issupplied therefrom to the counter 220 d. The counter 220 d counts thedetection pulses supplied from the rising edge detection part 220 c, andoutputs an end signal when its count value reaches the set number oftimes.

The flip-flop (JK-FF) 220 f includes J and K terminals and a CLEARterminal (CL). The end signal supplied from the counter 220 d is inputto the J terminal, and the K terminal is always maintained at a LOWlevel (“0”). The read-enabling signal (read gate) is input to the CLEARterminal (CL). By this structure, when the read-enabling signal (readgate) is made valid, the flip-flop (JK-FF) 220 f is enabled to outputthe above-mentioned second control signal that is made valid (Low) whenthe end signal is input and is made invalid (High) when theread-enabling signal (read gate) is made invalid. (In the drawing, acircle put by a gate represents logic inversion.) As previouslydescribed, the second control signal is supplied to the AND element 220b as a gate control signal.

The signals in the histogram operation enabling part 218 and the timingcontrol part 220 vary as shown in FIG. 9. In FIG. 9, when theread-enabling signal (read gate) is made valid to perform reading of themagneto-optical disk 10, the first counter 218 a of the histogramoperation enabling part 218 starts a count operation. When its countvalue reaches the start count value, the start pulse is output from thefirst counter 218 a to make valid the enabling signal (histo enable).When the enabling signal (histo enable) is made valid, the secondcounter 218 d starts a count operation. When its count value reaches theend count value, the stop pulse is output from the second counter 218 dto make invalid the enabling signal (histo enable).

Thereafter, the counter of the operation interval control part 220 a ofthe timing control part 220 starts a count operation. When its countvalue reaches the invalidity count value, the restart pulse is outputfrom the operation interval control part 220 a to make valid theenabling signal (histo enable) supplied from the histogram operationenabling part 218. Similarly, thereafter, when the count value of thesecond counter 218 d that starts the count operation when the enablingsignal (histo enable) reaches the end count value, the stop pulse makesthe enabling signal (histo enable) invalid. When the count value of thecounter of the operation interval control part 220 a that starts thecount operation when the enabling signal (histo enable) is made invalidreaches the invalidity count value, the restart pulse again makes validthe enabling signal (histo enable). When the number of times theenabling signal (histo enable) is made valid reaches the set number oftimes in process of repeating such operations, the second control signaloutput from the flip-flop (JK-FF) 220 f of the timing control part 220is made valid (Low) to inhibit the operation interval control part 220 afrom outputting the restart pulse.

By the above-described process, while the read-enabling signal (readgate) is valid, the enabling signal (histo enable) having a set timewidth (determined by the start and end count values) is output from thehistogram operation enabling part 218 for the set number of times atregular intervals (determined by the invalidity count value).

Every time the enabling signal that is made valid as described above issupplied to the counting part 204 during signal reproduction from themagneto-optical disk 10, the frequency of each of the levels of theequalized output data output from the digital equalizer 24 is counted bya corresponding one of the counters 204(1) through 204(64) of thecounting part 204. The count values of the counters 204(1) through204(64) are stored in the histogram memory 206 as a histogram of thefrequencies of the levels.

Since, in this embodiment, the data is recorded on the magneto-opticaldisk 10 in accordance with a PR (1, 1) waveform of the constraint length2, data to be reproduced (expected values) takes three values of 1, 0,and −1 (or 0, 1, and 2). If the reproduced signal (output of thelow-pass filter (LPF) 22 in FIG. 2), for instance, varies to draw acurve shown in FIG. 10, the frequencies of the levels of the equalizedoutput data from the digital equalizer 24 corresponding to dataquantized in the analog-to-digital converter 23 at clock timings(indicated by broken lines) are as shown in FIG. 11. The frequencies ofthe levels of the equalized output data are stored in the histogrammemory 206 as a histogram. In this case, although equalized output dataof levels corresponding to three expected values should have beendetected, the frequencies of the levels of the actual detected equalizedoutput data do not correspond to a discrete distribution of threediscrete values (wherein the number of discrete values is 3). Therefore,the recorded data cannot be reproduced accurately from the equalizedoutput data of such a state by the Viterbi detector 100.

Thus, the MPU 210 controls the tap coefficients of the digital equalizer24 by steps shown in FIGS. 12 and 13.

This operation may be performed by reading out test data prerecorded onthe magneto-optical disk 10, or by reading out data recorded on themagneto-optical disk 10 as necessary data. Further, this operation maybe performed at regular intervals when the power is turned on, a readerror is detected, and the recording medium (the magneto-optical disk10) is replaced with another.

In FIG. 12, initialization is performed (S1). In the initialization, asetting operation with respect to each counter, a setting operation withrespect to a target value for determining whether the number of discretevalues is proper, a setting operation with respect to a repetitionoperation, and a setting operation with respect to the digital equalizer(waveform equalizer) 24 are performed based on input information from auser. If values are set with respect to the necessary items as defaultvalues in the setting memory 216 and are not required to be modified,the initialization is not required for the items.

In the setting operation with respect to each counter, set are the startcount value corresponding to the start time of the histogram creationoperation, the end count value corresponding to the end time of thehistogram creation operation, the invalidity count value correspondingto the interval of the histogram creation operation, and the number oftimes the histogram creation operation is performed. In the settingoperation with respect to the target value for determining whether thenumber of discrete values is proper, set are a counter number and areference value (target value) for determination to specify a counter ofthe counting part 204 whose count value is subjected to thedetermination.

In the setting operation with respect to the repetition operation, ahistogram is created and the number of times the tap coefficients of thedigital equalizer 24 are adjusted based on the histogram is set. In thesetting operation with respect to the digital equalizer 24, set are theinitial values of the tap coefficients k−2, k−1, 0, k+1, and k+2, coarsechanges dckm2, dckm1, dck0, dckp1, and dckp2 in the tap coefficients,fine changes dfkm2 (<dckm2), dfkm1 (<dckm1), dfk0 (<dck0), dfkp1(<dckp1), and dfkp2 (<dckp2) in the tap coefficients, and the number oftimes the tap coefficients are coarsely adjusted.

After the initialization (S1) is completed, an internal counter is reset(cnt=0) (S2), and the read-enabling signal (read gate) is made valid(S3). With the read-enabling signal (read gate) being thus made valid, asignal is reproduced from the magneto-optical disk 10. During the signalreproduction, based on the set values (start count value, end countvalue, invalidity count value, and the number of times) set by theinitialization, the frequencies of the levels of the equalized outputdata from the digital equalizer 24 are counted for the set number oftimes at the regular intervals as previously described. The results arestored in the histogram memory 206 as a histogram.

After a given period of time passes, the read-enabling signal is madeinvalid (S4) and the signal reproduction from the magneto-optical disk10 is terminated.

Next, based on the histogram of the frequencies of the levels of theequalized output data created in the above-described manner, adetermination operation as to whether its number of discrete values is aproper value (S5) is performed. In this determination operation, thefrequencies of the levels of the histogram are read out from thehistogram memory 206. The frequency counted by the counter specified inthe initialization is selected from the numbers, and is compared withthe reference value (target value) set by the initialization.

For instance, in the case of a PR(1, 1) waveform, each peak value and acenter value of a reproduced waveform are data to be taken, andtherefore, correctly, levels between a reproduced signal and each of thepeak values (positive and negative) are not to be taken. Therefore, asshown in FIGS. 10 and 11, set in a predetermined register as thefrequencies subjected to the determination are the count value of alevel 40 (cntr 40) between a center level 32 of the reproduced signalthat varies from 0 to 63 in level and the maximum level 63 and the countvalue of a level 24 (cntr 24) between the center level 32 and theminimum level 0. Then, each of the count values (cntr 40 and cntr 24) iscompared with, for instance, a reference value “1”. If each of the countvalues (cntr 40 and cntr 24) is smaller than the reference value, it isdetermined that the number of discrete value (three in this case) isproper, and an OK flag is set to 1 (VALID). On the other hand, if one ofthe count values (cntr 40 and cntr 24) is greater than the referencevalue “1”, it is determined that the number of discrete values is notproper, and the OK flag is maintained to 0 (INVALID).

After the above-described determination operation is completed, it isdetermined whether the OK flag is set to VALID (S6). If the OK flag isnot set to VALID, it is determined that the number of discrete values isnot proper, and a recalculation operation of the tap coefficients of thedigital equalizer 24 is performed (S7). A description of therecalculation operation of the tap coefficients will be given later. Thetap coefficients stored in the setting memory 216 are replaced withvalues of the tap coefficients obtained by the recalculation operation.When the recalculation operation of the tap coefficients is completed,the internal counter is incremented (cnt=cnt+1) (S8), and it isdetermined whether a count value (cnt) of the internal counter reachesthe number of repetitions set in the initialization (S9).

Here, if the count value (cnt) of the internal counter does not reachthe number of repetitions, the read-enabling signal (read gate) is againmade valid (S3). Then, the values of the tap coefficients k−2, k−1, k0,k+1, and k+2 of the digital equalizer 24 are replaced with the values ofthe tap coefficients stored in the setting memory 216. With the tapcoefficients of the digital equalizer 24 being updated, the signalreproduction from the magneto-optical disk 10 is performed, and, as inthe above-described manner, a histogram of the frequencies of the levelsof the equalized output data from the digital equalizer 24.

The recalculation operation of the tap coefficients (S7) is performed inaccordance with steps shown in FIG. 13.

In FIG. 13, it is determined whether the count value (cnt) of theinternal counter is smaller than or equal to the value of the number ofcoarse adjustments set in the initialization (S71). If the count value(cnt) of the internal counter is smaller than or equal to the set valueof the number of coarse adjustments, the values of the tap coefficientsk−2, k−1, k0, k+1, and k+2 are read out from the setting memory 216(S72). The values are taken as km2, km1, k0, kp1, and kp2, respectively.By using these values of the tap coefficients and the coarse changesdckm2, dckm1, dck0, dckp1, and dckp2 set in the initialization, newvalues of the tap coefficients are calculated based on the followingexpressions:

km2=km2+dckm2

km1=km1−dckm1

k0=k0+dck0

kp1=kp1−dckp1

kp2=kp2+dckp2

The new values of the tap coefficients thus calculated are stored in thesetting memory 216 so that the tap coefficients are updated.

As previously described, the values of the tap coefficients of thedigital equalizer 24 are updated by the coarse changes so that ahistogram of the frequencies of the levels of the equalized output dataform the digital equalizer 24 is created, and the operations (S3, S4,S5, S6, S7, S8, and S9) including the determination as to whether thenumber of discrete values of the levels is proper are repeatedlyperformed. If it is determined in the recalculation operation of the tapcoefficients (S7) that the count value (cnt) of the internal counterreaches the set value of the number of coarse adjustments (that is, if“NO” in step S71 of FIG. 13) before the number of discrete valuesbecomes proper, the values km2, km1, k0, kp1, and kp2 of the tapcoefficients k−2, k−1, k0, k+1, and k+2 stored in the setting memory 216are read out (S74). By using these values of the tap coefficients andthe fine changes dfkm2, dfkm1, dfk0, dfkp1, and dfkp2 set in theinitialization, new values of the tap coefficients are calculated basedon the following expressions:

km2=km2+dfkm2

km1=km1−dfkm1

k0=k0+dfk0

kp1=kp1−dfkp1

kp2=kp2+dfkp2

The new values of the tap coefficients thus calculated are stored in thesetting memory 216 so that the tap coefficients are updated.

As previously described, the values of the tap coefficients of thedigital equalizer 24 are updated by the fine changes so that a histogramof the frequencies of the levels of the equalized output data form thedigital equalizer 24 is created, and the operations (S3, S4, S5, S6, S7,S8, and S9) including the determination as to whether the number ofdiscrete values of the levels is proper are repeatedly performed. If thenumber of discrete values becomes proper during the process, the OK flagis set to VALID (“YES” in step S6) to shift a read area to another zoneof the magneto-optical disk 10 (S10), and the same operations asdescribed above (S2 through S9) are performed.

If the number of discrete values in the histogram of the frequencies ofthe levels of the equalized output data becomes proper in each ofpredetermined zones, the values of the tap coefficients of the digitalequalizer 24 of each zone are stored in the nonvolatile memory 214 aftera predetermined process (P: provided by each apparatus) is performed(S11).

The values of the tap coefficients stored in the nonvolatile memory 214can be set in the digital equalizer 24 when the power is again turned onafter a shut-off.

If, in the above-described process, the number of discrete values in thehistogram of the frequencies of the levels of the equalized output databecomes proper before the count value (cnt) of the internal counterreaches the set value of the number of coarse adjustments, the values ofthe tap coefficients used in the calculation are stored in thenonvolatile memory 214 without the fine adjustment (steps S74 and S74 inFIG. 13). Further, if the count value (cnt) of the internal counterreaches the number of repetitions set in the initialization before thenumber of discrete values in the histogram of the frequencies of thelevels of the equalized output data becomes proper, the values of thetap coefficients obtained at that point are stored in the nonvolatilememory 214. In that case, readjustment can be performed, letting thevalues of the tap coefficients stored in the nonvolatile memory 214 bethe initial values.

Data reproduction is normally performed with the tap coefficients of thedigital equalizer 24 being set to values adjusted by the above-describedoperation. In this case, its equalized output data is detected at levelscloser to those at which the equalized output data is to be detectedcorrectly. As a result, the recorded data is reproduced from theequalized output data of such a state by the Viterbi detector 100 sothat more accurate reproduced data can be obtained.

For instance, the equalized output data obtained by reproducing arecorded signal of a PR (1, 1) waveform and varying as shown in FIG. 10is caused to be in a state shown in FIG. 14 by the above-describedadjustment of the tap coefficients of the digital equalizer 24. In thiscase, the equalized output data has a histogram of the frequencies ofits levels shown in FIG. 15. In this case, as described with respect tostep S5 of FIG. 12, the count values of the levels 40 and 24 (cntr 40and cntr 24) are smaller than the reference value (target value) “1”.Therefore, it can be determined that the proper number of discretevalues (3) is obtained.

The determination as to whether a proper number of discrete values isobtained from a histogram of the numbers of outputs of the levels of theequalized output data can also be performed based on the numbers ofdistribution areas of the histogram each of which areas has a standarddeviation less than or equal to a given value.

In the above-described embodiment, the description has been given of thecase where a signal is recorded in the PR(1, 1) waveform on therecording medium. However, the present invention is not limited to this.For instance, in the case of recording a signal in a PR(1, 2, 1)waveform of a constraint length 3, the levels of reproduced data can befive in number. In that case, the same operation can be performed,letting the number of discrete values being 5.

Further, the present invention is not limited to the case where a signalis recorded in a partial response (PR) waveform. The present inventioncan be applied to any system in which the levels of reproduced data canbe determined from a recording code.

Moreover, although the tap coefficients of the digital equalizer of atransversal type are adjusted in the above-described embodiment, thepresent invention can also employ a waveform equalizer of another typeand adjust its equalization characteristic. By adjusting a function thatworks the same way as a waveform equalizer (, such as reproduction poweradjustment or recording power adjustment), the same effect can beobtained.

Furthermore, since it can be determined from an obtained histogramwhether a recording medium is acceptable or not, data storagereliability can be increased by using an alternate sector for a badsector.

Next, a description will be given of another embodiment.

In this embodiment, expected values employed in the Viterbi detector 100are set, based on the information of a histogram created in the samemanner as in the above-described embodiment, in the read system unit 25of the magneto-optical disk unit shown in FIG. 1.

In this embodiment, the read system unit 25 has a structure shown inFIG. 16.

In FIG. 16, the read system unit 25, as in the above-describedembodiment (see FIG. 2), includes the low-pass filter (LPF) 22, theanalog-to-digital converter (ADC) 23, the digital equalizer 24, and theViterbi detector 100. Further, the read system unit 25 includes a risingedge detection circuit 33 and a trailing edge detection circuit 34 inaddition to an expected value setting unit 201 that replaces theequalizer control unit 200.

The expected value setting unit 201 has a structure shown in FIG. 17. InFIG. 17, the same elements as those of the equalizer control unit 200shown in FIG. 4 are referred to by the same numerals, and a descriptionthereof will be omitted.

In FIG. 17, the quantized data from the analog-to-digital converter 23is sequentially input to the rising edge detection circuit 33, whichdetects a rising edge of the reproduced signal based on a change in thequantized data. The quantized data from the analog-to-digital converter23 is sequentially input to the trailing edge detection circuit 34,which detects a trailing edge of the reproduced signal based on a changein the quantized data. A rising edge detection signal ONEDGE_DET fromthe rising edge detection circuit 33 and a trailing edge detectionsignal OFFEDGE_DET from the trailing edge detection circuit 34 aresupplied to the expected value setting unit 201.

Like the above-described equalizer control unit 200, the expected valuesetting unit 201 includes the decoder 202, the counting part 204, thehistogram memory 206, the MPU 210, the ROM 212, the nonvolatile memory214, the setting memory 216, the histogram operation enabling part 218,and the timing control part 220. The expected value setting unitadditionally includes a selector 221 and an AND circuit 222.

The rising edge detection signal ONEDGE_DET from the rising edgedetection circuit 33 and the trailing edge detection signal OFFEDGE_DETfrom the trailing edge detection circuit 34 are input to the selector221, which selects one of the rising edge detection signal ONEDGE_DETand the trailing edge detection signal OFFEDGE_DET. The enabling signal(histo enable) is supplied from the histogram operation enabling part218 to the counting part 204 via the AND circuit 222 that is controlledby the one of the detection signals selected by the selector 221.

The selector 221 can be fixed at a HIGH level (H) all the time based ona command from the MPU 210 irrespective of a state of the rising ortrailing edge detection signal ONEDGE_DET or OFFEDGE_DET. In this case,the AND circuit 222 is caused to be always in an enabled state by ahigh-level signal supplied from the selector 221, and the enablingsignal (histo enable) is supplied all the time to the counting part 204via the AND circuit 222.

That is, if the selector 221 is fixed at the HIGH level, the enablingsignal (histo enable) supplied from the histogram operation enablingpart 218 is made valid (for instance, HIGH) so as to have each of thecounters 204(1) through 204(64) of the counting part 204 perform a countoperation. If the selector 221 is not fixed at the HIGH level, each ofthe counters 204(1) through 204(64) of the counting part 204 performs acount operation on condition that the enabling signal (histo enable)supplied from the histogram operation enabling part 218 is made validwith one of the rising edge detection signal ONEDGE_DET and the trailingedge detection signal OFFEDGE_DET selected by the selector 221 beingmade valid (for instance, HIGH).

The trailing edge detection circuit 34 and the digital equalizer 24 havestructures shown in FIG. 18.

In FIG. 18, the digital equalizer 24, as in the above-describedembodiment, is formed of the transversal filter including a series offive cascaded delay elements (registers) T1 through T5, and has theadjustable tap coefficients k−2, k−1, k0, k+1, and k+2 for therespective delay elements.

The trailing edge detection circuit 34 includes a series of fourcascaded delay elements (registers) T6 through T9, a first comparator340, a second comparator 341, a third comparator 342, an OR element 343,and an AND element 344. The quantized data supplied from theanalog-to-digital converter (ADC) 23 is input to the first delay elementT6, and the quantized data input to the delay element T6 is shifted tothe delay elements T7, T8, and T9 sequentially in accordance with aclock signal.

The first comparator 340 compares the quantized data (A) set in thefourth delay element T9 at a time t-2 and the quantized data (B) set inthe third delay element T8 at a time t-1, and outputs a detection signalCMP1 that is valid when the quantized data (A) is larger than thequantized data (B) (A>B). The second comparator 341 compares thequantized data (A) set in the fourth delay element T9 at the time t-2and the quantized data (B) set in the third delay element T8 at the timet-1, and outputs a detection signal CMP2 that is valid when thequantized data (A) is smaller than or equal to the quantized data (B)(A≦B). The third comparator 342 compares the quantized data (A) set inthe third delay element T8 at the time t-1 and the quantized data (B)set in the second delay element T7 at a time t, and outputs a detectionsignal CMP3 that is valid when the quantized data (A) is larger than thequantized data (B) (A>B).

An AND signal AND1 of the detection signal CMP2 supplied from the secondcomparator 341 and the detection signal CMP3 supplied from the thirdcomparator 342 is generated in the AND element 344. An OR signal of thedetection signal CMP1 supplied from the first comparator 340 and the ANDsignal supplied from the AND element 344 is generated in the OR element343 to be output therefrom as the trailing edge detection signalOFFEDGE_DET of the trailing edge detection circuit 34.

The trailing edge detection circuit 34 of the above-described structuredetects a trailing edge of the reproduced signal based on the comparisonresults of the quantized data of the three consecutive timings (t-2,t-1, and t) provided by the synchronizing clock signal.

That is, if the quantized data set in the delay element T8 (t-1) issmaller than the preceding quantized data set in the delay element T9(t-2) (if the detection signal CMP1 supplied from the first comparator340 is valid), it is determined that the data corresponds to a trailingedge of the reproduced signal and the trailing edge detection signalOFFEDGE_DET is made valid. In this case, it makes no difference whetherthe quantized data set in the delay element T8 (t-1) is larger orsmaller than the next quantized data set in the delay element T7 (t).If, as shown in FIG. 19, the quantized data set in the delay element T8(t-1) is smaller than the next quantized data set in the delay elementT7 (t) (see a broken line), the quantized data set in the delay elementT8 represents a bottom value. If the quantized data set in the delayelement T8 (t-1) is larger than the next quantized data set in the delayelement T7 (t) (see a solid line), the quantized data set in the delayelement T8 (t-1) represents a value in a trailing edge of the reproducedsignal.

If the quantized data set in the delay element T8 (t-1) is larger thanor equal to the preceding quantized data set in the delay element T9(t-2) (the detection signal CMP2 supplied from the second comparator 341is valid) and is smaller than the next quantized data set in the delayelement T7 (t) (the detection signal CMP3 supplied from the thirdcomparator 342 is valid), the quantized data set in the delay element T8(t-1), as shown in FIG. 20, represents a peak value. Also in this case,it is determined that the data corresponds to a trailing edge of thereproduced signal, and the trailing edge detection signal OFFEDGE_DET ismade valid.

FIG. 21 shows a relation between the output of the digital equalizer 24and the trailing edge detection signal OFFEDGE_DET supplied from thetrailing edge detection circuit 34. In FIG. 21, vertical dotted linesrepresent the rising timings of the clock signal, in accordance withwhich each circuit operation is performed.

In an example shown in FIG. 21, the tap coefficients k−2, k−1, k0, k+1,and k+2 of the digital equalizer 24 are set to 0, 0, 1, 0, and 0,respectively. Therefore, an output data from the third delay element T3directly becomes the output data of the digital equalizer 24. Further,since the third delay element T8 of the trailing edge detection circuit34 provides a delay of the same amount as the third delay element of thedigital equalizer 24, an output data from the delay element T8 of thetrailing edge detection circuit 34 becomes identical to the output dataof the digital equalizer 24 at each timing.

In FIG. 21, the detection signal CMP3 supplied from the third comparator342 is made valid when the output data of the delay elements T9, T8, andT7 of the trailing edge detection circuit 34 become, for instance, (2,2, 1) (which is a state shown in FIG. 20). The detection signal CMP1supplied from the first comparator 340 is made valid and the detectionsignal CMP2 supplied from the second comparator 341 is made invalid whenthe output data of the delay elements T9, T8, and T7 become, forinstance, (2, 1, 0) (which is a state shown in FIG. 19). As a result,the trailing edge detection signal OFFEDGE_DET that is valid (HIGH)during the transition of the output data of the delay elements T9, T8,and T7 from (2, 2, 1) to (2, 1, 0) and to (1, 0, 0) is output from thetrailing edge detection circuit 34. Likewise, hereinafter, the trailingedge detection signal OFFEDGE_DET is made valid (HIGH) while the outputdata of the delay elements T9, T8, and T7 is in the states shown inFIGS. 19 and 20.

FIG. 22 shows a relation between the above-described output of thedigital equalizer 24 in the case shown in FIG. 21 and the trailing edgedetection signal OFFEDGE_DET. That is, the trailing edge detectionsignal OFFEDGE_DET is made valid at trailing edges of the reproducedsignal (corresponding to the output of the digital equalizer 24).

The rising edge detection circuit 33 has a structure show in FIG. 23.

In FIG. 23, the rising edge detection circuit 33 includes a series offour cascaded delay elements (registers) T10 through T13, a firstcomparator 330, a second comparator 331, a third comparator 332, an ORelement 333, and an AND element 334. The quantized data supplied fromthe analog-to-digital converter (ADC) 23 is input to the first delayelement T10, and is shifted to the delay elements T11, T12, and T13sequentially in accordance with the clock signal.

The first comparator 330 compares the quantized data (A) set in thefourth delay element T13 at the time t-2 and the quantized data (B) setin the third delay element T12 at the time t-1, and outputs a detectionsignal CMP4 that is valid when the quantized data (A) is smaller thanthe quantized data (B) (A<B). The second comparator 331 compares thequantized data (A) set in the fourth delay element T13 at the time t-2and the quantized data (B) set in the third delay element T12 at thetime t-1, and outputs a detection signal CPM5 that is valid when thequantized data (A) is larger than or equal to the quantized data (B)(A≧B). The third comparator 332 compares the quantized data (A) set inthe third delay element T12 at the time t-1 and the quantized data (B)set in the second delay element T11 at the time t, and outputs adetection signal CMP6 that is valid when the quantized data (A) issmaller than the quantized data (B) (A<B).

An AND signal AND1 of the detection signal CMP5 supplied from the secondcomparator 331 and the detection signal CMP6 supplied from the thirdcomparator 332 is generated in the AND element 334. An OR signal of thedetection signal CMP4 supplied from the first comparator 330 and the ANDsignal AND1 supplied from the AND element is generated in the OR element333 to be output therefrom as the rising edge detection signalONEDGE_DET of the rising edge detection circuit 33.

Like the above-described trailing edge detection circuit 34, the risingedge detection circuit 33 of the above-described structure detects arising edge of the reproduced signal based also on the comparisonresults of the quantized data of the three consecutive timings (t-2,t-1, and t) provided by the synchronizing clock signal.

That is, if the quantized data set in the delay element T12 (t-1) islarger than the preceding quantized data set in the delay element T13(t-2) (if the detection signal CMP4 supplied from the first comparator330 is valid), it is determined that the data corresponds to a risingedge of the reproduced signal and the rising edge detection signalONEDGE_DET is made valid. In this case, it makes no difference whetherthe quantized data set in the delay element T12 (t-1) is larger orsmaller than the next quantized data set in the delay element T11 (t).If, as shown in FIG. 24, the quantized data set in the delay element T12(t-1) is smaller than the next quantized data set in the delay elementT11 (t) (see a solid line), the quantized data set in the delay elementT12 represents a value in a rising edge of the reproduced signal. If thequantized data set in the delay element T12 (t-1) is larger than thenext quantized data set in the delay element T11 (t) (see a brokenline), the quantized data set in the delay element T12 (t-1) representsa peak value.

If the quantized data set in the delay element T12 (t-1) is smaller thanor equal to the preceding quantized data set in the delay element T13(t-2) (the detection signal CMP5 supplied from the second comparator 331is valid) and is smaller than the next quantized data set in the delayelement T11 (t), the quantized data set in the delay element T12 (t-1),as shown in FIG. 25, represents a bottom value. Also in this case, it isdetermined that the data corresponds to a rising edge of the reproducedsignal, and the rising edge detection signal ONEDGE_DET is made valid.

The rising edge detection circuit 33 of the above-described structureoutputs the rising edge detection signal ONEDGE_DET that is made valid(HIGH) at a rising edge of the reproduced signal (see FIGS. 24 and 25).

In the above-described expected value setting unit 201, an operation fordetermining the expected values employed in the Viterbi detector 100 isperformed under the control of the MPU 210.

This operation may be performed by reading out test data prerecorded onthe magneto-optical disk 10, or by reading out data recorded on themagneto-optical disk 10 as necessary data. Further, this operation maybe performed at regular intervals when the power is turned on, a readerror is detected, and the recording medium (the magneto-optical disk10) is replaced with another.

While the output of the selector 221 is valid (HIGH), as in the case ofthe equalizer control unit 200 of the above-described embodiment, thefrequencies of the levels of the equalized output data sequentiallysupplied from the digital equalizer is counted in the counting part 204.The counting results are stored in the histogram memory 206 as ahistogram representing a distribution of the frequencies of the levelsof the equalized output data. The MPU 210 reads out the data of thehistogram from the histogram memory 206 to calculate the average levelvalue of each concentration in the histogram, and defines the averagelevel value of each concentration as an expected value to be employed inthe Viterbi detector 100.

A specific description will now be given of expected values determinedin a case where data is recorded on the magneto-optical disk 10 inaccordance with PR(1, 1) of a constraint length 2. In the case of PR(1,1), there are three expected values corresponding to a peak value, acenter value (in each of trailing and rising edges), and a bottom valueof the reproduced signal.

If the output of the selector 221 is fixed at the HIGH level (H) basedon the command from the MPU 210, a histogram of the frequencies of thelevels of the entire reproduced signal is created. If the reproducedsignal has an ideal waveform without a droop or offset as shown in FIG.26, a histogram of the frequencies of the levels as shown in FIG. 27 iscreated. In this case, expected values corresponding to the peak value,center value (in each of the trailing and rising edges), and bottomvalue of the reproduced signal are defined as levels 47, 31, and 15,respectively.

Likewise, in the case of creating a histogram representing thefrequencies of the levels of the entire reproduced signal having awaveform slightly losing a shape compared with the ideal waveform asshown in FIG. 28 with the output of the selector 221 being fixed at theHIGH level (H), a histogram as shown in FIG. 29 is obtained. In thiscase, although each concentration of the histogram has a certain width,the average level values of the concentrations are levels 47, 31, and15. Therefore, as in the case of the reproduced signal of the idealwaveform, the expected values corresponding to the peak value, centervalue (in each of the rising and trailing edges), and bottom value ofthe reproduced signal are defined as levels 47, 31, and 15,respectively.

Further, in the case of creating a histogram representing thefrequencies of the levels of the entire reproduced signal having awaveform with droops as shown in FIG. 30 with the output of the selector221 being fixed at the HIGH level (H), a histogram as shown in FIG. 31is obtained. In this case, four concentrations of levels 40, 31, 25, and15 are formed in the histogram. Therefore, the expected valuescorresponding to the peak value, center value (at the rising edge),center value (at the trailing edge), and bottom value of the reproducedsignal are defined as levels 40, 31, 25, and 15, respectively.

In FIG. 30, a broken line indicates the ideal waveform of the reproducedsignal.

In the case of creating a histogram representing the frequencies of thelevels of the entire reproduced signal having a waveform with droops andoffsets as shown in FIG. 32 with the output of the selector 221 beingfixed at the HIGH level (H), a histogram as shown in FIG. 33 isobtained. In this case, four concentrations of levels 50, 41, 35, and 25are formed in the histogram. Therefore, the expected valuescorresponding to the peak value, center value (at the rising edge),center value (at the trailing edge), and bottom value of the reproducedsignal are defined as levels 50, 41, 35, and 25, respectively.

Moreover, in the case of creating a histogram representing thefrequencies of the levels of a trailing edge of the reproduced signalhaving a waveform with droops as shown in FIG. 34 (equal to the waveformshown in FIG. 30) with the selector 221 selecting the trailing edgedetection signal OFFEDGE_DET supplied from the trailing edge detectioncircuit 34, a histogram as shown in FIG. 35 is obtained. In this case,three concentrations of levels 40, 25, and 15 are formed in thehistogram. Therefore, the expected values corresponding to the peakvalue, center value (at the trailing edge), and bottom value of thereproduced signal are defined as levels 40, 25, and 15, respectively. Inthis case, since no histogram representing the frequencies of the levelsof a rising edge of the reproduced signal is created, the expected valuecorresponding to the center value at the rising edge of the reproducedsignal is not defined.

In the case of creating a histogram representing the frequencies of thelevels of a rising edge of the reproduced signal having a waveform withdroops as shown in FIG. 36 (equal to the waveform shown in FIG. 30) withthe selector 221 selecting the rising edge detection signal ONEDGE_DETsupplied from the rising edge detection circuit 33, a histogram as shownin FIG. 37 is obtained. In this case, three concentrations of levels 40,31, and 15 are formed in the histogram. Therefore, the expected valuescorresponding to the peak value, center value (at the rising edge), andbottom value of the reproduced signal are defined as levels 40, 31, and15, respectively. In this case, since no histogram representing thefrequencies of the levels of a trailing edge of the reproduced signal iscreated, the expected value corresponding to the center value at thetrailing edge of the reproduced signal is not defined.

As previously described, in the case where the reproduced signal has awaveform distortion (droop) in its trailing edge, the expected valuesdetermined from the sampled values (equalized output data) of the risingedge of the reproduced signal are identical to those determined from thereproduced signal of the ideal waveform (see FIGS. 26 and 27).

The expected values matching the distortion of the reproduced signal canalso be determined by considering, as described above, both of thehistogram of the frequencies of the levels of the reproduced signal inits trailing edge and the histogram of the frequencies of the levels ofthe reproduced signal in its rising edge.

The expected values thus determined are supplied to the Viterbi detector100 in accordance with the control of the MPU 210. The Viterbi detector100, at a time of data reproduction from the magneto-optical disk 10,determines data of maximum likelihood in accordance with the Viterbidecoding algorithm employing the comparison results of the suppliedexpected values and the equalized output data supplied from the digitalequalizer 24.

In the above-described embodiment, even if the reproduced signalincludes a steady distortion (droop) in a case where a RAD medium isused as the magneto-optical disk 10, the expected values to be employedin the Viterbi detector 100 are determined based on quantized dataobtained from the distorted reproduced signal in synchronism with asingle clock signal. Therefore, even if the reproduced signal from themagneto-optical disk 10 includes a steady distortion, the Viterbidetector 100 determines the data from the quantized data (equalizedoutput data) based on the expected values determined based on thedistorted waveform, thus achieving more accurate data reproduction.

Further, the expected values thus determined can be stored in thenonvolatile memory by the type of a recording medium to be used. In thiscase, expected values corresponding to the type of a recording medium tobe set are read out from the nonvolatile memory 214 to be supplied tothe Viterbi detector 100. Therefore, it is not necessary that such ahistogram as described above is created every time the type of arecording medium to be used is changed.

In the above-described embodiment, the description has been given of thereproduction of data recorded on the magneto-optical disk in accordancewith PR(1, 1), but the present invention is also applicable in a casewhere data is recorded in accordance with PR of a higher level.

FIG. 38 shows a reproduced signal of data recorded, for instance, inaccordance with constraint length 4 PR(1 1 0 0). In FIG. 38, a fine line(a square indicates a sampled value) represents the ideal waveform ofthe reproduced signal, and a bold line (a circle indicates a sampledvalue) represents a distorted waveform of the reproduced signal.

In FIG. 38, the sum (t+(t-1)) of data (t) and data (t-1) delayed by oneclock pulse represents the reproduced signal when the data varying asindicated on a time base t is recorded. Further, in the case ofconstraint length 4 PR(1 1 0 0), 16 expected values Ph0 through Ph15 aredetermined from quantized data of four consecutive timings (t, t-1, t-2,t-3) as shown in FIG. 39. For instance, in a case where the peak value,center value at the trailing edge, and bottom value of the reproducedsignal are defined as the levels 40, 25, and 15, respectively, as shownin FIGS. 34 and 35 and the center value at the rising edge of thereproduced signal is defined as the level 31 as shown in FIGS. 36 and37, the expected values Ph0, Ph1, and Ph3 are set to the level 15corresponding to the bottom value (0), the expected values Ph6 and Ph7are set to the level 25 corresponding to the center value (1) at thetrailing edge, the expected values Ph8 and Ph9 are set to the level 31corresponding to the center value (1) at the rising edge, and theexpected values Ph12, Ph14, and Ph15 are set to the level 40corresponding to the peak value (2).

In the case of using an RLL code, nonexistent data combinations areproduced to generate expected values that do not have to be defined. Inthe case of the (1, 7) RLL code, it is unnecessary to define theexpected values Ph2, Ph4, Ph5, Ph10, Ph11, and Ph13 due to D restrictionby the (1, 7) runlength-limited RLL code.

In an operation (ML) performed by the Viterbi detector 100 using theexpected values Ph0 through Ph15 thus determined, sampled values higherthan or equal to the level 40 are processed as the closest values to theexpected values of the level 40 (Ph12, Ph14, and Ph15).

The Viterbi detector 100 performing an operation in accordance with theViterbi decoding algorithm using the expected values thus determined hasa structure shown in FIG. 40.

In FIG. 40, the Viterbi detector 100 includes a path metric memory100(1), a branch metric calculation unit 100(2), an ACS unit 100(3), anda path memory 100(4).

The branch metric calculation unit 100(2) calculates a branch metricvalue corresponding to a difference between each expected value that isa correct value of the reproduced signal to be taken and a sampled valueof the reproduced signal. The ACS unit 100(3) adds the above branchmetric value to a path metric value of a preceding clock timing storedin the path metric memory 100(1), and compares each two of the pathmetric values obtained after the addition to select the smaller of thetwo. The selected path metric value is stored in the path metric memory100(1) as a new path metric value. As a result of the above-describedoperation, the path metric value is obtained as a cumulative value ofbranch metric values.

Selecting a smaller path metric value as previously describedcorresponds to selecting a state transition path in the Viterbi decodingalgorithm. That is, the ACS unit 100(3) always selects a path thatminimizes the path metric value.

Data corresponding to the path selected in the above-described manner issupplied from the ACS unit 100(3) to the path memory 100(4). In the pathmemory 100(4), the data corresponding to each selected path issequentially shifted, and, during the process, data corresponding toeach unselected path is sequentially discarded so that datacorresponding to a surviving path is output from the path memory 100(4)as output data. Thus, by recording data in a partial response (PR)waveform and detecting data of maximum likelihood (ML) by employing theViterbi detector 100, data can be reproduced with high accuracy from themagneto-optical disk 100 on which data is recorded with high density.

In the above-described embodiments, the equalizer control unit 200 (seeFIGS. 2 and 4) and the expected value setting unit 201 (see FIGS. 16 and17) are separately treated. However, an equalizer control function andan expected value setting function can be provided in a single unit bysharing parts that the two units have in common.

INDUSTRIAL APPLICABILITY

As previously described, a data reproduction control method andapparatus, and an optical disk unit according to the present inventioncan reproduce data stably and more properly even when a reproducedsignal varies locally due to a noise or includes a steady distortion.Therefore, the present invention is suitably applied to an apparatusthat is desired to reproduce data stably from a recording medium such asan optical disk, a magneto-optical disk, or a magnetic disk on whichdata is recorded with high density.

What is claimed is:
 1. A data reproduction control method controlling adistribution characteristic of quantized data by adjusting a waveformequalization characteristic at a time of performing quantization andwaveform equalization on a signal reproduced from a recording mediumrecorded with data in accordance with a given recording code, wherein:frequencies of levels of the quantized data are counted; and thewaveform equalization characteristic is adjusted so that a distributionof the counted frequencies of the levels approaches a distribution thatis to be obtained based on the given recording code.
 2. The datareproduction control method as claimed in claim 1, wherein thefrequencies of the levels of the quantized data are counted at givenintervals during a period of signal reproduction from the recordingmedium.
 3. The data reproduction control method as claimed in claim 1,wherein: a given number of periods of signal reproduction from therecording medium are set; and the frequencies of the levels of thequantized data are counted and the waveform equalization characteristicis adjusted in each of the periods of signal reproduction.
 4. The datareproduction control method as claimed in claim 3, wherein the waveformequalization characteristic is adjusted by a given amount in each of theperiods of signal reproduction.
 5. The data reproduction control methodas claimed in claim 4, wherein the waveform equalization characteristicis adjusted by a first given amount in first given ones of the periodsof signal reproduction and is adjusted by a second given amount in therest of the periods of signal reproduction, the second given amountbeing less than the first given amount.
 6. The data reproduction controlmethod as claimed in claim 1, wherein a discrete state in thedistribution of the counted frequencies of the levels is determined, andthe waveform equalization characteristic is adjusted so that saiddiscrete state matches a discrete state of the distribution that is tobe obtained based on the given recording code.
 7. A data reproductioncontrol apparatus controlling a distribution characteristic of quantizeddata by adjusting a waveform equalization characteristic of a waveformequalizer when a signal reproduced from a recording medium recorded withdata in accordance with a given recording code is subjected tooperations in quantization means and the waveform equalizer, the datareproduction control apparatus comprising: counting means for countingfrequencies of levels of the quantized data; and equalizationcharacteristic adjustment means for adjusting the waveform equalizationcharacteristic of the waveform equalizer so that a distribution of thefrequencies of the levels obtained in said counting means approaches adistribution that is to be obtained based on the given recording code.8. The data reproduction control apparatus as claimed in claim 7,comprising counting control means for causing said counting means toperform an operation at given intervals in a period of signalreproduction from the recording medium.
 9. The data reproduction controlapparatus as claimed in claim 7, comprising: number setting means forsetting a given number of periods of signal reproduction from therecording medium; and control means for causing said counting means andequalization characteristic adjustment means to perform operations ineach of the periods of signal reproduction of the given number set insaid number setting means.
 10. The data reproduction control apparatusas claimed in claim 9, wherein the equalization characteristicadjustment means adjusts the waveform equalization characteristic by agiven amount in each of the periods of signal reproduction.
 11. The datareproduction control apparatus as claimed in claim 10, wherein: thewaveform equalizer is of a transversal type; and said equalizationcharacteristic adjustment means adjusts tap coefficients of the waveformequalizer by given amounts.
 12. The data reproduction control apparatusas claimed in claim 10, comprising adjustment number setting means forsetting a number of adjustments of the waveform equalizationcharacteristic, wherein said equalization characteristic adjustmentmeans comprises: first adjustment means for adjusting the waveformequalization characteristic by a first given amount in first ones of theperiods of signal reproduction, a number of the first ones being equalto the number of adjustments set in the adjustment number setting means;and second adjustment means for adjusting the waveform equalizationcharacteristic by a second given amount in the rest of the periods ofsignal reproduction, the second given amount being less than the firstgiven amount.
 13. The data reproduction control apparatus as claimed inclaim 12, wherein: said waveform equalizer is of a transversal type;said first adjustment means adjusts tap coefficients of the waveformequalizer by first given values; and said second adjustment meansadjusts the tap coefficients of the waveform equalizer by second givenvalues less than the first given values.
 14. The data reproductioncontrol apparatus as claimed in claim 7, wherein: said equalizationcharacteristic adjustment means comprises discrete state determinationmeans for determining a discrete state in the distribution of thecounted frequencies of the levels; and the waveform equalizationcharacteristic is adjusted so that the discrete state determined by saiddiscrete state determination means matches a discrete state of thedistribution that is to be obtained based on the given recording code.15. An optical disk unit having a reproduction system includingprocessing means for performing quantization and waveform equalizationon a signal reproduced from an optical disk medium recorded with data inaccordance with a given recording code, and maximum likelihood detectionmeans for determining, in accordance with a maximum likelihood detectionprocess corresponding to the recording code, reproduced data fromwaveform-equalized quantized data obtained in the processing means, theoptical disk unit comprising: counting means for counting frequencies oflevels of the quantized data; and equalization characteristic adjustmentmeans for adjusting a waveform equalization characteristic in thewaveform equalization so that a distribution of the frequencies of thelevels obtained in said counting means approaches a distribution that isto be obtained based on the given recording code.